NKT cells are a population of regulatory T cells that is known to be able to potently modulate immune responses. The antigens that control NKT cell activation are lipids and glycolipids presented by CD1d molecules. A remarkable characteristic of NKT cells is that many are autoreactive to self antigens. This allows them to become activated even when there is no foreign antigenic challenge, and may be critical for their endogenous immunoregulatory effects. The auto-antigens recognized by human NKT cells have not been identified, and little is known about how their presentation is regulated. Here we will: i) characterize the endogenous ligands presented by human CD1d molecules, and analyze which ones are antigenic for NKT cells;ii) follow up on our preliminary results, which indicate that an intercellular lipid messenger called lyso- phosphatidylcholine (LPC) that is produced under inflammatory conditions, is a self antigen that can be recognized by many human NKT cells;iii) investigate the cellular and molecular requirements for CD1d-mediated auto-antigen presentation to NKT cells. These studies will provide new information about the lipid characteristics that are important for binding to human CD1d molecules and for recognition by NKT cells, and will explore how NKT cell activation is linked to broader physiological processes of inflammation through recognition of bio-active lipid mediators as self antigens, and will provide critical insights into the mechanisms by which CD1d- mediated auto-antigen presentation is regulated by APCs. PUBLIC HEALTH RELEVANCE: NKT cells are autoreactive regulatory T cells that recognize lipids and glycolipids as antigens presented by CD1d molecules. The molecular identity of the auto-antigens recognized by human NKT cells remains unknown. In this grant we will: i) identify constitutively presented cellular ligands that are recognized by human NKT cells;ii) investigate lipid messengers as putative inducible auto-antigens that may directly link NKT cell activation to inflammatory responses;and iii) analyze cellular and molecular requirements for auto-antigen presentation by CD1d.